This invention relates to an electroforming apparatus and a process for using that apparatus to prepare an electroform, more specifically, the invention relates to a female mandrel with an interior fluid passageway for preparing electroforms therein when an electrolytic solution flows through the passageway while the mandrel is cathodic.
The fabrication of hollow, relatively thin articles from a metal by the process of electroforming is widely practiced in industry. Electroforms are used in many areas including printing, xerographing, and photocopying. Electroforms are also used in the printing of currency. One of the main advantages of the use of electroforms in printing type processes is the ability to produce many exactly identical copies. The quality and detail achieved with electroforming is superior to other techniques because of electroforming's ability to replicate exactly the design, or lack thereof, without any or with very minimal imperfections.
Basically, electroforming or electrodeposition is the process of depositing a substance, such as a metal, onto a conducting mold using electrical current. Electroforming processes known in the art submerse the conducting mold or mandrel into a bath of electrolytic solution through which a voltage drop exists.
The voltage drop results from connecting the mandrel to one terminal of a DC voltage source while a second terminal of the DC voltage source supplies the electrolytic solution with a current. The current flows through the conducting electrolytic solution from one terminal to the second terminal. This current flow involves a voltage drop, i.e., the voltage drop is the voltage developed across the electrolytic solution (conductor) during the flow of electrical current through the resistance of the electrolytic solution.
An electrolytic solution or electrolyte is a solution or other conducting medium in which electric current flows. The flow of electric current in the solution is caused by the migration of ions through the solution.
One of the early methods of electroforming is disclosed in U.S. Pat. No. 3,464,898. The outside surface of a plastic mandrel is coated with an electrically conductive material. The mandrel is then coupled to an electrode and immersed in an electrolytic or plating solution for electrodeposition. An electroform forms on the outside surface of the mandrel. The mandrel with an electroform thereon is removed from the plating solution and rinsed. The electroform is then separated from the mandrel by use of a solvent, or by heating and volatilizing the plastic material.
The following features, steps, and/or elements are present in some or all of the prior art electroforming devices and processes:
(1) the mandrel is immersed in a bath of electrolytic solution where both the inside and outside surface are in fluid contact with the bath; PA1 (2) the working surface on the mandrel is typically an outer (male) surface, although U.S. Pat. No. 5,160,4241 discloses a female mandrel that is rotated and submersed into an electroforming bath to form an electroform; PA1 (3) electroforming on a male surface requires compressive stresses; PA1 (4) to create compressive stresses during electroforming, a sulfamic electrolyte such as sulfamate is used; PA1 (5) the anode is soluble because insoluble anodes counteract with the sulfamic electrolyte thereby forming undesirable anode byproducts (but the sulfamic electrolytes are required to produce the necessary compressive stresses); PA1 (6) soluble anodes solubilize thereby unequally changing (increasing) the distance between the cathode and the anode as the anode is consumed; PA1 (7) the mandrel or anode is rotated while in the electrolytic solution to compensate for the inherent nonuniform anode to cathode distance resulting from the use of soluble anodes which cause the anode to cathode distance to change as the anode is consumed; PA1 (8) coatings are needed on the mandrel to block deposition where deposition is not desired; and PA1 (9) to release the electroform from the male mandrel, one or more of the following three concepts are taken advantage of: (a) a difference in the thermal coefficients of expansion between the electroform and the mandrel; (b) the internal stress of the electroform; and (c) a hysteresis effect during cooling. PA1 (1) the electrolytic solution tank in which the bath is given is open to receive the mandrel thereby allowing contaminants and impurities to freely enter the bath of electrolytic solution; PA1 (2) unpleasent, noxious and/or harmful vapors and fumes are given off by the open tank of electrolytic solution; PA1 (3) a large quantity of electrolytic solution is needed to submerse the entire mandrel therein; PA1 (4) numerous working parts are needed to move the mandrel in and out of the electrolytic solution (i.e., a complex mechanical mechanism); PA1 (5) in the male mandrel prior art, the working surface of the electroform is its outer surface, while the working surface of the mandrel upon which the electroform is formed is its outer surface, therefore the working surface of the electroform is not created on the working surface of the mandrel--as a result the working surface of the electroform does not have the controllable characteristics of the working surface of the mandrel; instead the inner, never used, surface of the electroform has these characteristics; PA1 (6) compressive stresses as are necessary on a male mandrel require the use of certain electrolytic solutions such as sulfamate which must be vented due to the fumes, requires chemical additives, has a small stability temperature range, creates undesirable by products when used with insoluble anodes, and is expensive; PA1 (7) the solubilization of the anode creates distance differences between the cathode and the anode; PA1 (8) numerous working parts and connections such as electrical brushes are needed for the required rotation of either the anode or the cathode to offset the nonuniform anode to cathode distance; PA1 (9) insoluble anodes counteract with the electrolytic solution thereby forming undesirable anode byproducts; and PA1 (10) soluble anodes solubilize thereby changing (increasing) the distance between the cathode and the anode as the anode is consumed.
The thermal coefficient of expansion difference between the mandrel material and the electroform occurs on a male mandrel where the mandrel has a higher thermal coefficient of expansion, such as 13.times.10.sup.-6 in./in..degree. F. for an aluminum mandrel, than that of the electroform, such as 8.times.10.sup.-6 in./in..degree. F. for a nickel electroform, and where the internal stress of the electroform is not too tensile. The result from the mandrel having a higher thermal coefficient of expansion than the electroform is a larger decrease in the diameter of the mandrel than the decrease in the diameter of the electroform during cooling after the formation of the electroform. This larger diameter decrease by the mandrel compared to the electroform causes a gap to form between the electroform and the mandrel. The parting gap, if sufficiently large, will allow the electroform to slide off of the outside surface of the male mandrel.
Electroform internal stress control is useful in separation of the electroform from the mandrel, particularly with smaller electroforms. Internal stress control involves control of the internal stresses of the electroform to facilitate removal of the electroform. Electrolytic deposits naturally have tensile internal stresses; however, for an electroform to form on a male mandrel, compressive internal stresses rather than tensile internal stresses are required.
Using various techniques, including the use of additives such as saccharin in the electrolytic solution, compressive internal stresses are created. During electroforming on a male mandrel, i.e., plating of the male mandrel, one or more of the cation specics, for example Ni.sup.+2, which are the electroform materials in the electrolytic solution are reduced and adhere to the mandrel, based upon the mandrel being sufficiently cathodic while the electrolytic solution is anodic, thereby forming the electroform. During cooling, cold shock occurs causing additional stress to be applied to the electroform. The result of this cold shock is the expansion of the electroform as it releases its bond with the mandrel and the electroform takes on a new size (slightly larger in the case of a compressively stressed electroform made on a male mandrel).
Hysteresis effect occurs where the hot male mandrel with an electroform therearound is cooled, such as in a cool water bath. The outside electroform will cool first for two reasons: (1) it is on the outside and in direct contact with the coolant, and (2) the electroform typically has a higher thermal conductivity than the mandrel, such as where the electroform is nickel and the mandrel is stainless steel. The result is the electroform wants to shrink before the mandrel. However, the mandrel is preventing the electroform from shrinking so the electroform must yield, i.e. stretch or expand. Then several seconds later, the mandrel cooling catches up and it shrinks. The electroform recovers some, but some hysteresis or residual stretching remains. The result is a parting gap between the faster shrinking electroform which was forced to stretch and the slower shrinking mandrel which eventually shrinks more.
The above mentioned features, steps, and/or elements of the prior art present a number of disadvantages, including but not limited to the following: